1. Field of the Invention
The present invention relates to a mesoporous titania bead and the preparation method thereof.
2. Description of the Related Art
After industrial revolution, fossil fuel consumptions grew dramatically accompanying with the development of science, and resulted in fossil fuel exhaustion and environmental damages. For sustainable survival, the development of renewable and alternative energy was the ultimate goal of the world. In all alternative energies, solar energy caught people's attention because it was abundant and clean, and many companies had invested in the associated research and development.
Solar cell, also called photovoltaic cell, was a device for converting light energy into electrical energy. However, lots of energy was consumed during the manufacture of solar cells, so it was still a challenge for solar cells to reach grid parity. At present, most commercial solar cells were silicon solar cells, in which monocrystalline silicon solar cells and multicrystalline silicon solar cells had a cell efficiency of 18% and 17%, respectively. But the cost of silicon solar cells was high because pure crystalline silicon materials were widely used in semiconductor industry. Materials generally used for non-silicon thin-film solar cells were cadmium telluride (CdTe) or copper indium gallium diselenide (CIGS, CuInGaSe), in which the former material was mainly used by First Solar for manufacturing solar cells with the lowest price per watt in all commercial solar cells, but cadmium contamination was a concerned issue; and the latter material could be used for manufacturing stable solar cells with high efficiency and long life span, but the complicated element composition caused low yield rate.
A solar cell promising for dramatically reducing electricity cost was dye-sensitized solar cell (DSC), which was published on Nature in 1991 (B. Oregan and M. Grätzel, “A Low-Cost, High-Efficiency Solar-Cell Based On Dye-Sensitized Colloidal TiO2 Films,” Nature, 353 (6346), 737-740, 1991). DSC had advantages that it cost less and could be applied to flexible applications. Comparing with silicon solar cells, it was less influenced by incident angle and increased temperature, so the DSC was very competitive and potential to lead the trend of the next generation solar cells. There had been many commercial DSCs in the market, for example, Sharp had manufactured a DSC having a high cell efficiency of 10.4%. Generally, DSC had a shorter life span and lower cell efficiency; however, if these disadvantages were overcome, it would be the most widely used solar cell in the future.
In DSCs, the photoelectrode was important for loading dye molecules and transferring electrons, and it was the key to decide cell efficiency. The main material for producing the photoelectrode was titania nanoparticles. Titania (TiO2) was a stable, non-toxic material with high refractive index (n=2.4-2.5), and widely used in our daily life, such as in white pigment, tooth paste, cosmetics or food. The naturally occurring titania had three main crystal phases: rutile, anatase and brookite, in which the rutile titania was the most stable crystal phase in view of thermodynamics; but the anatase titania was suitable for cell applications because it had a greater energy band gap and a higher conduction band, so the anatase titania could reach a greater quasi-Fermi energy level and open circuit voltage under the same electron concentration, thereby achieving a better cell efficiency.
Regarding with the morphology of titania, the titania nanoparticles (NPs) was widely applied to DSCs because it had a high specific surface area which was able to absorb a large amount of dye. However, NPs did not have an oriented structure, and the electrons immigrated in random directions, so the electron collection efficiency was limited. In addition, the particle size of NPs was too small to produce effective visible light scattering and good light harvesting. Therefore, many strategies had been taken to solve this problem, for example, M. Zukalova et al. produced an oriented particle by polymer template method and the resulted cell efficiency was higher than that of non-oriented particle by 1.3% (Nano Letters, 5 (9), 1789-1792, 2005); J. M. Macak et al. prepared a TiO2 nanotube with high aspect ratio by anodization (Angewandte Chemie-International Edition, 44 (14), 2100-2102, 2005) and J. R. Jennings et al. produced a photoelectrode from TiO2 nanotube and titanium substrate, giving a electron collection efficiency of nearly 100% (Journal of the American Chemical Society, 130 (40), 13364-13372, 2008), which demonstrated that tubular or linear structures provide a better diffusion direction for electrons; and K. Shankar et al. proved that when a glass substrate was used instead, the cell efficiency would reached 6.1% (Nano Letters, 8 (6), 1654-1659, 2008). Nevertheless, the structure of nanotube and the like did not provide sufficient dye-loading, so the other structures derived from nanoparticles were still under research and development.
Another way to solve the low dye-loading problem was to use the structure called TiO2 beads (see D. H. Chen et al., Advanced Materials, 21 (21), 2206, 2009 and Y. J. Kim et al., Advanced Materials, 21 (36), 3668, 2009). The TiO2 bead with submicron-meter size had the following advantages: (1) this bead dramatically increased light harvesting efficiency because its size was large enough for Mie scattering, so the light route in the photoelectrode lengthens and dye loading increased; (2) this bead had a large surface area, which helped dye loading; (3) TiO2 bead had regular mesopores that increased electron transfer and helped mass transfer of electrolyte. However, this two-layer photoelectrode was only applied to rigid DSCs, not introduced into flexible dye-sensitized solar cells (FDSCs). This was because there were less contacts between large size TiO2 beads and the substrate, so the photoelectrode was not well-attached on the substrate, and this highlighted the disadvantage of FDSCs. In recent studies, the best cell efficiency of the flexible low-temperature glass DSCs using TiO2 beads was 6.3% (S. H. Jang et al., Electrochemistry Communications, 12 (10), 1283-1286, 2010). TiO2 was not used in the general flexible plastic substrates because the plastic substrates could only be processed at 150° C. or less and they could not bear the high temperature treatment for removing organic compounds and sintering TiO2 beads on the traditional rigid substrates (about 450° C.). Therefore, the DSCs using TiO2 beads had low electron collection efficiency and reduced cell efficiency.